MEDIUM: proxy: switch conf.name to cebis_tree

MEDIUM: proxy: switch conf.name to cebis_tree

This is used to index the proxy's name and it contains a copy of the
pointer to the proxy's name in <id>. Changing that for a ceb_node placed
just before <id> saves 32 bytes to the struct proxy, which is now 3112
bytes large.

Here we need to continue to support duplicates since they're still
allowed between type-incompatible proxies.

Interestingly, the use of cebis_next_dup() instead of cebis_next() in
proxy_find_by_name() allows us to get rid of an strcmp() that was
performed for each use_backend rule. A test with a large config
(100k backends) shows that we can get 3% extra performance on a
config involving a static use_backend rule (3.09M to 3.18M rps),
and even 4.5% on a dynamic rule selecting a random backend (2.47M
to 2.59M).